British Society November meeting features presentation on Low Temperature Cure Epoxy Powder Coatings.

At the November meeting of the British Society, "Low Temperature Cure Epoxy Powder Coatings" was discussed by Dr. Mike Robins, speaking on behalf of Klaus Hoffman of Huntsman Advanced Materials.

Dr. Robins detailed the main advantages and disadvantages of powder technology. Benefits include: 100% solids coatings (no VOCs), powder coatings are economical with little waste, they require low investment, are suitable for "line" production, are available as interior and exterior coatings, and offer ease of formulation for certain special effects. The disadvantages mentioned were: high curing temperatures of typically 170[degrees]C or higher, having to use relatively thick films compared with other industrial coatings, and the ability to only coat relatively small items.

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Types and chemistries used in conventional powder coatings were then discussed, including pure epoxy based for high performance applications with high temperature and chemical resistance properties, such as for pipe coatings, coating reinforcing bars to be set in concrete, and automotive under-bodies. These require a latent curing agent and are only intended for use internally or in areas that will not be exposed to UV light. According to the speaker, epoxy and acid functional polyesters with epoxies are used to achieve cure.

Dr. Robins explained that exterior systems are fully weatherable for all exterior applications, and use polyester plus crosslinkers such as TGIC, PT910, PT912, and Primid XL-552. Polyurethane acrylic technology can also be used, but this accounts for less than 5% of coatings used.

He next discussed the potential benefits of using low-temperature curing powders, such as those suitable for coating on temperature-sensitive substrates, such as wood and plastic. These include saving energy with lower oven temperatures, using conventional temperatures for faster curing, and obtaining better quality of film cure over large/thick "heat sink" objects where object temperature is hard to reach.

Before concentrating on hardeners that would facilitate low-temperature cure, Dr. Robins described typical hardeners currently in use. He discussed the suitability of such hardeners as DICY, modified DICY with aromatics, aromatic amines (DDS), imidazoles, carboxyl functional polyesters, phenolic curing agents, Novalac hardeners, and anhydride hardeners. He stated that the critical consideration for lower-temperature cured powder systems would be the need for additional accelerator, and typical imidazole-based accelerators can decrease storage stability. Ideal latency and cure profiles of traditional powders and those based on the new hardener were discussed and compared. Dr. Robins noted that the new hardener has improved latency, improved storage stability, and is based on a heterocylic type amide in which it can open the epoxy groups and self-catalyze. This eliminates the need for additional accelerator.

The new hardener can be made to cure down as low as 110-120[degrees]C with careful formulation and extruder conditions. It also requires a low dosage of two to five parts per hundred resin. The objective in the development was to make a resin for use in EP/PE pipe coatings with improved, or at least equal, properties. It must also pass accelerated storage conditions at 50[degrees]C for one month. Most properties with the new hardener were improved, including a very good bend angle result. The wet soak adhesion was slightly worse.

In conclusion, Dr. Robins noted that the new hardener, XB 3123, has greater flexibility, and improved storage stability and latency than other crosslinkers. A crystalline solid with a melting point of 180[degrees]C, it is produced and registered for use in Europe. With the new hardener, powder coatings can be formulated with cure temperatures as low as 110-120[degrees]C. In addition, it is possible to formulate with conventional accelerators which will decrease expense.